msm: dcvs: add frequency transient control to sysfs

It is useful to be able to toggle at runtime whether
particular DCVS points are used as transient levels. This
adds a sysfs node per core which when read returns a list
of the currently enabled DCVS transient levels. Writing
the frequency of a DCVS transient level toggles whether it
is enabled or not.

Change-Id: I69ebb6974e97efa832798047259b9acdfd08aa7c
Signed-off-by: Steve Muckle <smuckle@codeaurora.org>